C.M.
Sometimes I feel like that with my stepdaughter. We were able to get her more at our house than her mom's (which is good!) and relations between her mother and us are fine.
Still, at our house she has responsibilities, boundaries, bedtimes, good nutrition, manners, etc. At her mom's there is no bedtime, she can eat whatever she wants, no manners, no responsibilities. When she comes back from mom's she's always messed up. It takes a day or two to get her back.
Shared custody can feel a bit like banging your head against a wall! I have dreaded the time when she turns into a teen if some things don't change.
You know, the world WILL knock your child on their butt if they are not prepared. I think the best you can do is let your son know you are there for him to teach him what he needs to know, but you will not GIVE it to him. He may need to spread his wings and "crash" before he's ready to make the change.
I've had to stand by and watch my stepdaughter "fail" at something before she's ready to listen. I hate doing it--I hate seeing her sad or frustrated. But you know, those are the moments that make the biggest impression.
Loving your son is all you can do. And you may need to have a heart-to-heart with him. Ask him what he wants out of life. Sometimes that's all it takes! My SD is only 10 but when she gets really bad I ask her how she wants her life to be. Does she want a life with no privileges?
Sometimes kids just need a wake-up call--but also need to know they can get what they want and you can help them.
Good luck! And you're not alone!